Transgenic or recombinant plants are of increasing interest because of the potential to control phenotypic traits as well as to produce large quantities of commercially useful products. Plants have been employed to overproduce heterologous proteins and in principle can produce a wide range of products, including high value proteins and certain pharmaceuticals. Transgenic plants with visually attractive phenotypes are particularly desirable as a source of economic benefit to horticulturists and to florist retailers, while transgenic agronomic crops with enhanced production traits are of economic benefit to food producers and consumers.
Genetically modified plants for agricultural products are already on the market, including herbicide, insect and virus resistant crop plants. Some of the better-known crops engineered for herbicide resistance include soybeans, maize, rapeseed, sugar beet, rice and cotton. Maize, potatoes, tomatoes and cotton have been modified for insect resistance.
Food source plants can be engineered to improve traits that affect nutritional value; for example, elevated iron in rice and wheat, higher amino acid content in potatoes, seedless fruits and increased carotenoids in rice and tomatoes. Recent efforts have turned to produce recombinant plants with maximal desired plant product at a selected harvest time (patent publication 20030093836, May 15, 2003) or to significantly increase a desired expressed product by targeting protein product accumulation in a targeted tissue (patent publication 20040117874, Jun. 17, 2004). Of particular interest are plants engineered to increase oil production; for example, canola oil, which is considered more healthful than trans fats and oils.
Cytokinins play a role in many growth and developmental processes in plants, such as apical dominance, cell differentiation, flowering, fruit set and ripening, leaf senescence and seed germination. The effects of cytokinins on plants can be exploited for agricultural and horticultural purposes through either exogenous application of cytokinin or endogenous manipulation of cytokinin metabolism. In Hatiora gaetneri for example, flower bud number can be more than doubled in response to a spray application of synthetic cytokinins (Boyle, 1995). The efficacy of exogenous spray applications is limited however, because flowers and leaves do not readily absorb cytokinins and movement of cytokinins within the plant is limited.
Alternatively, it has been reported that endogenous levels of cytokinins can be modified by integrating the ipt gene into the plant genome. The ipt gene encodes the enzyme isopentenyl transferase, which catalyzes the rate-limiting step in cytokinin biosynthesis. A number of promoters, including those inducible by heat, wounding, or light, have been used to drive ipt gene expression. Unfortunately, most of the resulting ipt transgenic plants exhibit morphological abnormalities since overproduction of cytokinins interferes with so many developmental processes (Gan and Amasino, 1997).
Transgenic plants that overproduce cytokinins tend to show reduced stature, release of apical dominance, changes in vascular development, and in some cases, inhibited root growth (Ainley, et al., 1993) In one study, Li, et al. (1992) fused the ipt gene to the auxin-inducible SAUR promoter. This promoter is primarily active in elongating tissue and SAUR-ipt plants expressed elevated levels of cytokinins in these tissues. SAUR-ipt plants displayed reduced stature, increased axillary bud development, reduced root initiation and growth, and exhibited complex and variable changes in senescence.